For a long time the concept of copper flexible was just stacking' together bunches of foils and connecting the same.
Thereafter, rivetting at the ends was started. Further improved by process of soldering & brazing, but the loss of energy was never considered/assessed. Later on, as the cost of energy increased tremendously, the technique of hot pressure fusion welding process was put in use which is the latest trend and resulting into substantial savings in energy cost. AMA is using the same technique.